Ligand Effects on Structures and Spectroscopic Properties of Pyridine

ARTICLE pubs.acs.org/JPCA

Ligand Effects on Structures and Spectroscopic Properties of Pyridine-2-aldoxime Complexes of Re(CO)3þ: DFT/TDDFT Theoretical Studies Ting-Ting Zhang, Jian-Feng Jia, Ying Ren, and Hai-Shun Wu* School of Chemistry and Materials Science, Shanxi Normal University, Linfen, 041004, China

bS Supporting Information ABSTRACT: The series of novel rhenium(I) tricarbonyl mixed-ligand complexes Re(X)(CO)3(N∧N) (N∧N = pyridine-2-aldoxime; X = Cl, 1; X = CN, 2; and X = CtC, 3) has been investigated theoretically to explore the ligand X effect on their electronic structures and spectroscopic properties. The contribution of the X ligand to the highest occupied molecular orbital (HOMO) and HOMO-1 decreases in the order of 3 > 1 > 2, in line with the π-donating abilities of the X: CtC > Cl > CN. The reorganization energy (λ) calculations show that 1 and 3 will result in the higher efficiency of organic light-emitting diodes than 2. The lowestlying absorptions of 1 and 3 can be assigned to the {[dxz, dyz(Re) þ π(CO) þ π(X)] f [π* (N∧N)]} transition with mixing metal-to-ligand, ligand-to-ligand, and X ligand-to-ligand charge transfer (MLCT/LLCT/ XLCT) character, whereas this absorption at 354 nm (H-1 f L) of 2 is assigned to {[dxz, dyz(Re) þ π(CO) þ π(N∧N)] f [π* (N∧N)]} transition with MLCT/LLCT/ILCT (intraligand charge transfer). Furthermore, the absorptions are red-shifted in the order 2, 1, and 3, with the increase of π-donating abilities of X ligands. The solvent effects cause red shifts of the absorption and emission spectra with decreasing solvent polarity.

1. INTRODUCTION For decades, there has been continuous interest in electroluminescent materials, and it is still a very promising research area nowadays because of their potential application in organic lightemitting diodes (OLEDs).1 In this aspect, numerous complexes containing d6 heavy metal ions such as rhenium(I),2 ruthenium(II),3 osmium(II),4 rhodium(III),5 and iridium(III)6 have been extensively studied by various spectroscopic and electrochemical techniques. Because of the strong spinorbital coupling of the heavy transition metals, the triplet metal-to-ligand charge transfer (3MLCT) excited state can emit effectively molecular phosphorescence by borrowing the intensity of the singlet MLCT excited state. Thus, theoretically, OLEDs prepared by using phosphorescence heavy metal complexes would display efficiency 34 times better than those based on fluorescent materials.7 Especially, Re(I) tricarbonyl complexes as one kind of the phosphorescent materials have been focused on greater interest for their photophysical properties, solar energy conversion, OLED, and potential applications on the basis of their emission character.8 For example, Re(X)(CO)3(N∧N) type complexes (N∧N, a bidentate N,N0 -chelating ligand such as 2,20 -bipyridine or 1,10-phenanthroline, and X = Cl, Br) have been used in a variety of processes including photochemical reduction of CO2,9 chemoluminescence,10 and luminescent probe in curing of epoxy r 2011 American Chemical Society

resins.11 Recently, a series of novel tricarbonyl rhemium complexes, which can be used in diagnostic and therapeutic radiopharmaceuticals, such as [Re(CO)3(tp)2Cl], [Re(CO)3(bpzm)Cl], and [Re(CO)3(bdmpzm)Cl] (tp = 1,2,4-triazolo-[1,5-a]pyrimidine, bpzm = bis(pyrazol-1-yl)methane, and bdmpzm = bis(3,5-dimethylpyrazol-1-yl)methane) have been reported by Machura and co-workers.12 Moreover, their photophysical and photochemical behavior depends on the nature of low-lying excited states, which can be controlled by structural variations of the ligands X and N∧N or by the medium.13 Costa etc. have first synthesized and investigated the luminescent properties of Re(X)(CO)3(N∧N) (N∧N = pyridine-2-aldoxime; X = Cl, Br),14 but they could not interpret the spectroscopic property from an electronic structure point of view. The oxime complexes are important because they may be used as the labeling of antibodies15 for their relative long lifetimes and high stability. In this study, an in-depth theoretical understanding of the electronic structures and spectroscopic properties of the Re(I) tricarbonyl mixed-ligand complexes Re(X)(CO)3(N∧N) (N∧N = pyridine-2-aldoxime; X = Cl, 1; X = CN, 2; and X = CtC, 3) were carried out using density functional theory (DFT) and Received: January 27, 2011 Published: March 22, 2011 3174

dx.doi.org/10.1021/jp200872b | J. Phys. Chem. A 2011, 115, 3174–3181

The Journal of Physical Chemistry A

ARTICLE

Figure 1. Optimized geometries of 13 at the PBE1PBE/LANL2DZ/6-31G(d) level.

time-dependent density functional theory (TDDFT). The aim of the theoretical investigation is to establish the relationship between the spectra and the π-donating abilities of the X ligand, as well as the effect of metal on enhancing the luminescence quantum yields. Moreover, the solvent effects on the spectra are researched with the polarized continuum model (PCM).

2. COMPUTATIONAL DETAILS All calculations have been performed using the Gaussian 03 program package.16 The geometry of the singlet ground state and the lowest triplet state of 13 was optimized by the DFT17 method. A hybrid HartreeFock/density functional model approach based on the PerdewBurkeErzenrhof (PBE) functional,18 referred to as PBE1PBE, where the HF/DFT exchange ratio is fixed a priori to 1/4, was used to optimize the ground state geometries, and the unrestricted PBE1PBE (UPBE1PBE) method was used to optimize the excited state geometries. On the basis of the optimized ground and excited state geometries, the absorption and emission properties in different media were calculated by TDDFT19 at the PBE1PBE hybrid functional level associated with the PCM.20 Spinorbital coupling is not included in the current TDDFT method, and it influences the excitation energies for d(Re)-joined transitions,21 whereas it has a negligible effect on the transition character of these complexes. Hence, although TDDFT cannot exactly estimate the excitation energies for d(Re)-joined transitions, it can still provide a reasonable spectral feature for our investigated complexes. This kind of theoretical approach has been proven to be reliable for transition-metal complex systems.22 In the calculations, the quasi-relativistic pseudopotentials of Re atom proposed by Hay and Wadt23 with 14 valence electrons were used, and a “double-ξ” quality LANL2DZ basis set was adopted for Re atom, and the 6-31G(d)24 basis set was adopted for other atoms. To explain the rationality of the PBE1PBE method and LANL2DZ/6-31G(d) basis set, complex 1 was selected to do the calculation test with different functionals and basis sets. Tables S1 and S2 (Supporting Information) show that the geometry parameters and the excited energies of the lowest-lying absorptions obtained by LANL2DZ/6-31G(d) basis set but different functionals, and the most accurate results can be obtained from PBE1PBE. The calculated results obtained by other larger basis set including LANL2DZ/6-311G(d) and LANL2DZ/6-311þG(d) are all consistent with the basis set LANL2DZ/6-31G(d) (Tables S3 and S4 in the Supporting Information), so the LANL2DZ/6-31G(d) basis set can give satisfied results, and it is good for saving computational resources.

Table 1. Main Optimized Ground State Geometry Parameters of 13, Together with the Experimental Data of 1 1 S0

T1

2 S0

experimental

3 T1

S0

T1

bond length (Å) ReC(1)

1.926 1.973

1.922

1.927 1.941 1.924 1.966

ReC(2)

1.919 1.930

1.939

1.920 1.927 1.916 1.927

ReC(3)

1.914 1.945

1.913

1.960 1.968 1.966 2.033

ReN(1) ReN(2)

2.185 2.067 2.142 2.055

2.183 2.154

2.192 2.211 2.193 2.176 2.147 2.089 2.143 2.078

ReCl

2.475 2.426

2.481

ReC(4)

2.105 2.099 2.098 2.033 bond angle (deg)

N(1)ReN(2) 73.9

72.8

74.0

N(1)ReCl

87.7

82.0

83.1

N(1)ReC(4)

73.6

74.7

73.5

75.9

83.6

89.9

84.2

86.8

3. RESULTS AND DISCUSSION 3.1. Geometries and Frontier Molecular Orbital Compositions in the Ground State. The optimized structures of 13 are

shown in Figure 1, and the main geometry parameters together with the experimental data of 1 are listed in Table 1. As depicted in Figure 1, the Re(I) atoms adopt a distorted octahedral coordination geometry and bond distances and angles around the Re center are typical for fac-[Re(X)(CO)3(N∧N)] complexes,25 with the N(1)ReN(2) angle being characteristically small (Table 1). The ReX bond is slightly tilted toward the N∧N ligand, and the fac-Re(CO)3 unit is a nearly regular trigonal pyramid with ∼90° angles between the CO ligands. The optimized structural parameters of 1 are in general agreement with the experimental values, and the slight discrepancy comes from the crystal lattice distortion existing in the real molecules. The partial frontier molecular orbital compositions and energy levels of 13 are listed in Tables 24, respectively. It can be found that the three highest occupied molecular orbitals (HOMOs) and HOMO-1s have a mixed Re/CO/X character with different contributions. Table 2 shows that the HOMO of 1 is mainly composed of 48.3% d(Re), 21.0% π(CO), and 23.4% p(Cl), while that of 3 has a similar composition (Table 4). However, with respect to the HOMO of 2, the contribution from the CN ligand is decreased to 8.2%, the proportion of Re is increased to 56.2%, and the N∧N ligand is increased to 11.2% (Table 3). The variation trend of HOMO-1 3175

dx.doi.org/10.1021/jp200872b |J. Phys. Chem. A 2011, 115, 3174–3181


ARTICLE

Table 2. Frontier Molecular Orbital Compositions (%) in the Ground State for Complex 1 at the PBE1PBE Level contribution (%) orbital

energy (eV)

Re

N∧N

CO

Cl

main bond type

Re component ∧

72

0.8742

71

1.1617

95.2

π*(N∧N)

70

2.4630

80.7

π*(N∧N)

69

6.5419

48.3

21.0

23.4

d(Re) þ p(Cl) þ π(CO)

12.6 dxz þ 30.9 dyz

68

6.6460

47.1

18.1

25.1

d(Re) þ p(Cl) þ π(CO)

31.2 dxz þ 10.0 dyz

67

7.2083

67.9

27.4

d(Re) þ π(CO)

11.7 dx2y2 þ 53.4 dxy

66

7.5420

65 63

8.0830 8.8718

23.1

24.7

p(Re) þ π*(CO) þ π*(N N)

47.5

HOMOLUMO energy gap

76.0

21.7

π(CO) þ p(Cl)

19.9

64.0 57.1

π(N∧N) þ p(Cl) π(CO) þ p(Cl)

18.2 12.1

6.69 dz2


energy (eV)

Re

N∧N

CO

CtN

main bond type

Re component

∧

71

0.4798

20.7

26.1

36.5

p(Re) þ π*(N N) þ π*(CO)

70

1.0807

24.1

28.3

42.9

p(Re) þ π*(N∧N) þ π*(CO)

69

1.1892

89.2

π*(N∧N)

68

2.4333

82.4

π*(N∧N)

67

6.7211

56.2

12.5

21.5

8.2

d(Re) þ π(N∧N) þ π(CO)

9.0 dxz þ 45.1 dyz

66

6.8209

57.1

11.2

20.4

9.2

d(Re) þ π(N∧N) þ π(CO)

46.4 dxz þ 6.1 dyz

65

7.1846

67.9

d(Re) þ π(CO)

11.8 dx2y2 þ 53.1 dxy

60

9.2752

54.2

58

9.8178

72.2

HOMOLUMO energy gap

27.3 16.5

π(N∧N) þ π(CN) π(N∧N) þ π(CO)

11.8


energy (eV)

Re

N∧N

CO

CtC

main bond type

Re component

∧

71

0.2100

18.5

10.8

59.8

p(Re) þ π*(N N) þ π*(CO)

70 69

0.8364 1.0684

22.8

23.5 94.6

49.0

p(Re) þ π*(N∧N) þ π*(CO) π*(N∧N)

68

2.2742

67

6.0346

43.7

20.1

30.7

d(Re) þ π(CO) þ π(CtC)

12.8 dxz þ 26.9 dyz

66

6.1094

41.2

16.2

33.6

d(Re) þ π(CO) þ π(CtC)

17.4 dxz þ 11.3 dyz

65

6.9580

67.1

28.0

d(Re) þ π(CO)

11.8 dx2y2 þ 52.5 dxy

64

7.2289

63 62

7.4664 7.8910

π*(N∧N)

82.3 HOMOLUMO energy gap

63.7 29.8 25.0

23.5 46.6

23.0

π(N∧N) þ π(CtC)

16.4

d(Re) þ π(CO) þ π(CtC) d(Re) þ π(N∧N) þ π(CtC)

49.0

is similar with HOMO. Therefore, it can be concluded that the contribution of X ligand to the HOMO and HOMO-1 decreases in the order of 3 > 1 > 2, in line with the π-donating abilities of X ligands: CtC > Cl > CN. However, HOMO-2s of 13 are of d(Re) (over 67%) and π(CO) character. Both lowest unoccupied molecular orbitals (LUMOs) and LUMOþ1s are predominantly π* antibonding orbital of the N∧N ligand,

22.4dyz

whereas the LUMOþ2s of 13 have the p(Re), π* (N∧N), and π*(CO) contributions. In addition, different X ligand has a direct effect on the orbital energy of HOMO, which is decreasing in the order CtC > Cl > CN. It is consistent with a decreasing trend in the π-donating abilities of X ligands CtC > Cl > CN, and it indicates that the ability of losing an electron from the metal d 3176



ARTICLE

orbital becomes more difficult in 2 and easy in 3 with respect to 1. The calculated results indicate that the X ligand with a stronger π-donating ability can increase the energy level of the HOMO more significantly than those of the LUMO resulting in narrower HOMOLUMO energy gaps, which then lead to change the absorption and emission spectra. 3.2. Ionization Potential (IP), Electron Affinity (EA), and Reorganization Energy. The DFT-calculated IP, EA, reorganiTable 5. IPs, Electronic Affinities, Extraction Potentials, Reorganization Energies, and Spin Densities for Each Molecule (in eV) spin density of cation (%) IP(v) IP(a) HEP λhole

Re

N∧N

3CO

Cl 0.27

1 2

7.82 8.04

7.49 7.78

7.19 7.49

0.63 0.64 0.01 0.55 0.76 0.01

0.08 0.08

3

7.27

7.01

6.74

0.53 0.54 0.02

0.06

CN

CtC

0.17 0.42

spin density of anion (%) EA(v) EA(a) EEP λelectron

Re

N∧N 3CO

1

1.32

1.59

2.02

0.70

0.01 0.95

2

1.39

1.63

2.11

0.72

0.01 1.06 0.06

3

1.14

1.36

1.58

0.44

0.02 0.97

Cl

CN CtC

0.04 0.02 0.04

0.01 0.01

zation energy (λ), hole extraction potential (HEP) and electron extraction potential (EEP), and the spin densities for the three complexes are listed in Table 5. The IP and EA can be either for vertical excitations (v, at the geometry of the neutral molecule) or adiabatic (a, optimized structure for both the neutral and charged molecule). In addition, HEP is the energy difference from M (neutral molecule) to Mþ (cationic), using Mþ geometric structure in calculation, and EEP is the energy difference from M to M (anionic), using M geometric structure in the calculation. 26 As shown in the cation spin densities in Table 5, the three complexes share the common features of having accessible Rebased oxidations and N∧N-based reductions. The calculated IPs decrease in the following order 2 > 1 > 3, which implies that the difficulties of hole injection from the hole-transporting layer gradually decreases, and this trend is also consistent with the order of HOMO energy levels. The unpaired spin density is totally on the N∧N ligand (over 90%), and the change of EA is relatively smaller, which is in accord with the analysis of the LUMO composition and energy. To value the charge-transfer rate and balance, reorganization energy (λ) was calculated for the studied molecules. According to the Marcus/Hush model,27 the charge (hole or electron) transfer rate k can be expressed by the following formula: π 1=2 V 2 λ λ exp k¼ ¼ A exp ð1Þ λkb T 4kb T 4kb T p where T is the temperature, kb is the Boltzmann constant, λ is the reorganization energy, and V is the coupling matrix element between the cation and the molecules, which is dictated by the overlap of orbitals. Obviously, the reorganization energy in the charge transfer process is very important. The reorganization energy λ (herein the internal reorganization energy obtained by ignoring any environmental relaxation and changes) for hole transfer can be expressed as follows28 λhole ¼ λ0 þ λþ ¼ ðE0 E0 Þ þ ðEþ Eþ Þ ¼ IPðvÞ HEP

Figure 2. Schematic description of internal reorganization energy for hole transfer.

ð2Þ

As illustrated in Figure 2, E0 and Eþ represent the energies of the neutral and cation species in their lowest energy geometries,

Table 6. Absorptions of 13 in Methanol Calculated According to the TDDFT/PBE1PBE Method, Together with the Experimental Value of 1 complex 1

2

3

transition

|CI| (coeff)

E (eV/nm)

oscillator

assign

H-1 f L

0.679

3.25/382

0.110

MLCT/LLCT/XLCT

H-4 f L

0.560

4.62/268

0.104

MLCT/ILCT/XLCT

HfLþ1

0.313

H-6 f L H-3f L þ 1

0.431 0.340

5.57/223

0.044

MLCT/ILCT/LLCT/XLCT ILCT/XLCT

H-1 f L

0.658

3.50/354

0.172

MLCT/LLCT/ILCT

H-2f L þ 2

0.362

4.97/249

0.144

HfLþ3

0.301

H-9 f L

0.347

H-7 f L

0.245

H-1 f L

0.686

2.94/422

0.079

MLCT/LLCT/XLCT

H-4 f L HfLþ1

0.469 0.299

4.24/292

0.168

MLCT/LLCT/ILCT/XLCT MLCT/LLCT/XLCT

H-3f L þ 3

0.346

5.99/207

0.067

H-5f L þ 1

0.318

λexp (nm) 377

MLCT/LLCT/XLCT

MLCT/LLCT/ILCT MLCT/LLCT/ILCT

5.92/210

0.081

ILCT/LLCT LLCT/XLCT

MLCT/LLCT/ILCT ILCT/XLCT

3177



ARTICLE

respectively, while E0* and Eþ* represent the energies of the neutral and cation species with the geometries of the cation and neutral species, respectively. In this way, λ for electron transfer can be expressed as follows λelectron ¼ λ0 þ λ ¼ ðE0 E0 Þ þ ðE E Þ ¼ EEP EAðvÞ

ð3Þ

Emitting layer materials need to achieve hole and electron injection and transport balance, and a low reorganization energy is necessary for an efficient charge transport process. As shown in Table 5, the λhole values of 1 and 2 are smaller than the λelectron values, which suggests that the hole-transfer rate is better than the electron-transfer rate. In contrast, 3 has a smaller λelectron and thus better electron-transporting performance, while the differences between λhole and λelectron for 1 and 3 (0.07 and 0.09, respectively) are smaller than 2 (0.17), which can greatly improve the charge-transfer balance, thus further enhancing the device performance of OLEDs. The above analysis results present that the device performance can be easily changed by varying different X ligands. So, it is a key point toward the development of novel materials of OLEDs. 3.3. Absorptions in Methanol Media. The calculated absorption energies associated with their oscillator strengths, the main

Figure 3. Simulated absorption spectra of 13 in methanol solution.

configurations, and their assignments, as well as the experimental result of 1 are given in Table 6. The fitted absorption curves are shown in Figure 3. Figure 4 displays the energy levels of molecular orbital involved in electronic transitions of 13, which can intuitively understand the transition process. The best agreement with the experimental spectra (377 nm) of 1 was obtained by using the PBE1PBE functional in conjunction with the PCM solvent model in methanol. Table 6 shows the lowest-lying distinguishable singlet f singlet absorption band at 382 nm for 1 and is almost pure H-1 f L excitation for the transition. Table 2 shows that H-1 of 1 is composed of (31.2 dxz þ 10.0 dyz) Re, 18.1% π(CO), and 25.1% p(Cl), whereas the LUMO is dominantly localized on π* (N∧N) with 80.7%. Thus, the lowest-lying absorption at 382 nm for 1 can be assigned to {[dxz, dyz(Re) þ π(CO) þ p(Cl)] f [π* (N∧N)]} transition with mixing MLCT/LLCT/XLCT. Also, the lowest-lying absorption band of 3 has a similar transition path to that for 1 at 422 nm. However, for 2, from the above frontier molecular orbital discussion, the H-1 includes a 11.2% contribution from the N∧N ligand and little X contribution, so this absorption at 354 nm (H-1 f L) can be described as {[dxz, dyz(Re) þ π(CO) þ π(N∧N)] f [π* (N∧N)]} transition with MLCT/LLCT/ ILCT (intraligand charge transfer). Moreover, the MLCT transition plays an important role in the excitation resulting from the Re being the main proportion in HOMO-1. So, it can be found that the absorption intensity of 2 is stronger than 1 and 3 from Figure 3. On experiment, the lowest-lying absorption of 1 at 377 nm is also attributed to MLCT, which basically agrees with our calculated results. By comparing the absorptions of 13 at 382, 354, and 422 nm, it is found that the lowest-lying absorptions are red-shifted in the order 2, 1, 3, which is consistent with a increasing trend in the π-donating abilities of X ligands CN < Cl < CtC. The second distinguishable absorption bands at 268, 249, and 292 nm dominate these higher energy absorption bands for 13, respectively. As shown in Table 6, the transition of H-4 f L and H f Lþ1 contribute to the 268 nm absorption of 1, which can be ascribed as {[d(Re) þ π(N∧N) þ p(Cl)] f [π* (N∧N)]} and {[d(Re) þ π(CO) þ p(Cl)] f [π* (N∧N)]} with MLCT/ ILCT/XLCT and MLCT/LLCT/XLCT transition characters,

Figure 4. Diagrams of the molecular orbital related to the absorptions of 13. 3178



ARTICLE

respectively. For 3, the H-4 f L and H f L þ 1 are also responsible for the 292 nm absorption with the transition {[d(Re) þ π(N∧N) þ π(CO) þ π(CtC)] f [π* (N∧N)]} and {[d(Re) þ π(CO) þ π(CtC)] f [π* (N∧N)]} and the characters of MLCT/ILCT/LLCT/XLCT and MLCT/LLCT/ XLCT. With respect to 2, the absorption at 249 nm has a different transition character from that of 1 and 3. Table 6 shows that the excitation of H-2 f L þ 2 and H f L þ 3 is the dominant contribution to the absorption band of 2 at 249 nm, and it can be described as a {[d(Re) þ π(CO) þ π(N∧N)] f [π* (N∧N) þ π* (CO)]} with the mixed character of MLCT, LLCT, and ILCT. The highest-lying absorptions at 200250 nm for 1 and 3 are mainly attributed to {[d(Re) þ π(CO) þ π(N∧N) þ π(X)] f [π* (CO) þ π* (N∧N)]} transition with MLCT/ILCT/LLCT/XLCT, while 2 is assigned to {[π(CO) þ π(N∧N) þ π(X)] f [π* (CO) þ π* (N∧N)]} transition with ILCT, LLCT, and XLCT character. 3.4. Geometries in the Lowest-Lying Triplet Excited State and Emissions in Methanol Media. The lowest triplet states T1 of 13 have been optimized by the UPBE1PBE method, and selected geometrical parameters are depicted in Table 1. The calculated results reveal that geometrical parameters of 13 have small differences from the ground state structures, and the three complexes show a similar variation trend. All of the ReC bond lengths are relatively longer than those in the ground state, but the ReN bond lengths are strengthened, which indicates that the CO ligands tend to break away from the Re atom, but the N∧N ligands are getting closed in the excited state. This is attributed to the minor changes that result from the excitation as well as electrons transfer from the ReCO bonding orbital to the π* (N∧N) orbital upon excitation. The phosphorescence energies of 13 in methanol media was calculated from the energy difference between the ground singlet and the triplet states in the triplet state optimized geometry (Table 7). As compared with the experimental phosphorescence value of ReCl(CO)3(bpy),29 it is more accurate than the excitation energies calculated by TDDFT (Table S5 in the Supporting

Information). However, the TDDFT can still provide a reasonable spectral feature for transition-metal complex systems.30 The frontier molecular orbital compositions responsible for the emissions are compiled in Table 8. As shown as Table 7, the lowest energy emissions of 13 are mainly from the transitions of LUMO f HOMO. With respect to 1, the emission at 696 nm originates from the 3{[dxz(Re) þ π(CO) þ p(Cl) þ π(N∧N)] [π* (N∧N)]} excited state with 3MLCT, 3LLCT, 3XLCT, and 3 ILCT character. The emission of 3 at 737 nm has a transition character similar to that of 1. For 2, Table 8 shows that the contribution of π(CN) to the HOMO orbital is only 2.5%, which is lower than in other complexes, therefore, indicating that the 3 XLCT transition hardly plays an important role in the emission of 2. To intuitively understand the emission transition, the plots of frontier molecular orbitals related to emissions of 13 are presented in Figure 5. The emission results indicate that the 3 XLCT transition composition decreases in the order 2 < 1 < 3 along with the same order of the π-donating abilities of X ligands CN < Cl < CtC, and the emissions of 1 and 3 are blueshifted as compared with 2. Moreover, 1 and 3 emit light in the

Table 7. Calculated Emission Energies of T1 and Their Transition Character for 13 transition |CI| (coeff) E (eV/nm)

character

1 2

LfH LfH

0.731 0.529

1.78/696 0.54/2313

3

3

LfH

0.693

1.68/737

3

MLCT/3LLCT/3XLCT/3ILCT MLCT/3LLCT/3ILCT

3

Figure 5. Single electron transitions for the emissions of T1 states for 13 calculated at TDDFT/PBE1PBE level.

MLCT/3LLCT/3XLCT/3ILCT

Table 8. Frontier Molecular Orbital Compositions (%) in the Excited State for 13 contribution (%) orbital

energy (eV)

L

2.8307

H

6.1847

L H

3.5442 6.2628

L

2.6387

H

5.6826

Re

N∧N

CO

X

main bond type

1 π*(N∧N)

76.9 42.8

19.7

26.1

87.0 57.4

16.6

18.9

dyz(Re) þ p(Cl) þ π(CO) þ π(N∧N)

2 12.2

2.5

π*(N∧N) dyz(Re) þ π(N∧N) þ π(CO)

3 π*(N∧N)

77.4 41.5

12.2

17.1 3179

26.4

dyz(Re) þ π(CtC) þ π(CO) þ π(N∧N) dx.doi.org/10.1021/jp200872b |J. Phys. Chem. A 2011, 115, 3174–3181


ARTICLE

Table 9. Calculated Values for the Absorptions (λex) and Emissions (λem) in Different Solutions solvent polarity

CH3OH 6.6

CH3COCH3 5.4

CHCl3

C6H5CH3

4.4

2.4

C6H12

emission spectra of 13 have a red shift with the decrease of polarity form methanol to cyclohexane. These theoretical studies can provide constructive information in discovering new efficient phosphorescent materials.

0.1

λex(1) (nm) λex (2) (nm)

382 354

385 356

404 370

429 387

437 392

’ ASSOCIATED CONTENT

λex (3) (nm)

422

428

450

478

485

λem(1) (nm)

696

701

716

735

742

λem (3) (nm)

737

743

762

792

802

Supporting Information. The geometry parameters and the excited energies of the lowest-lying absorptions obtained by LANL2DZ/6-31G(d) basis set but different functionals for 1 (Tables S1 and S2), the calculated results obtained by different basis sets (Tables S3 and S4), and calculated emission energies of ReCl(CO)3(bpy) with two different methods (Table S5). This material is available free of charge via the Internet at http://pubs. acs.org.

visible range, but 2 occurs in the near-infrared region. There is no experimental assignment for the emissions so the calculations extend the experimental results. 3.5. Solvent Effect on the Absorption and Emission Spectra. Different solvents can cause different excitation energies due to the polarity. The different solvent effects for the absorption and emission energies of 1 and 3 are evaluated using the PCM method, as shown in Table 9. Both the absorption and the emission spectra have a red shift with the decrease of polarity from methanol to cyclohexane. The change trend is different from the solvent effect on the spectra of the complexes [Re(R2bpy)(CO)3X] (R = H, t-Bu; X = Cl, OTf, CtCpyRe(R2bpy)(CO)3).31 A red shift is observed on the absorption spectra of the complexes [Re(R2bpy)(CO)3X] with decreasing polarity, while a blue shift is observed on the emission spectra. The change rule will provide useful guidance for future experiments. The solvent effect can be explained by the multiparametric method of Kamlet and Taft,32 in which the absorption and emission energies are correlated with different solvent properties according to equation, one of the most extensively applied ν ¼ ν0 þ aR þ bβ þ pðπ þ dδÞ where ν h0 is the value of the absorption and/or emission energies in a reference solvent, R is an index of the solvent's ability to act as a hydrogen bond donor toward a solute, and β is a measure of the ability of a bulk solvent to act as a hydrogen bond acceptor, π* is an index of the solvent polarity, and δ is polarizability correction. The parameters a, b, p, and d can be retrieved through a multiparametric fitting on various solvents.

4. CONCLUSION The present work investigated the ground and excited state geometries, absorption, and emission properties of three tricarbonyl Re(I) complexes with pyridine-2-aldoxime and X (Cl, CN, CtC) ligands theoretically. The X ligand with a stronger π-donating ability can increase the energy level of the HOMO more significantly than those of the LUMO resulting in narrower HOMOLUMO energy gaps. The differences between λhole and λelectron of 1 and 3 are smaller and can greatly improve the charge transfer balance, thus further enhancing the device performance of OLEDs. The absorption energy increases in the order 3 < 1 < 2 along with the reverse order of the increasing π-donating ability of X ligands, and the MLCT transition plays an important role for 2. The emission results indicate that the 3XLCT transition composition decreases in the order 2 < 1 < 3, in line with the order of the π-donating abilities of X ligands CN < Cl < CtC, and the emissions of 1 and 3 are blue-shifted as compared with 2. The absorption and

bS

’ AUTHOR INFORMATION Corresponding Author

*Tel/Fax: þ86 0357 2052468. E-mail: [email protected].

’ ACKNOWLEDGMENT We are grateful for the financial aid from No. 20871077 and the National Nature Science Foundation of Shanxi of China (No. 2010011012-1). ’ REFERENCES (1) (a) Wand, Y.; Herron, N.; Grushin, V. V.; LeCloux, D. D.; Petrov, V. A. Appl. Phys. Lett. 2001, 79, 449. (b) Xin, H.; Li, F. Y.; Shi, M.; Bian, Z. Q.; Huang, H. C. J. Am. Chem. Soc. 2003, 125, 7166. (c) Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.; Kamatani, J.; Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.; Okada, S.; Hoshino, M.; Ueno, K. J. Am. Chem. Soc. 2003, 125, 12971. (2) (a) Dominey, R. N.; Hauser, B.; Hubbard, J.; Dunham, J. Inorg. Chem. 1991, 30, 4754. (b) Sacksteder, L.; Lee, M.; Demas, J. N.; DeGraff, B. A. J. Am. Chem. Soc. 1993, 115, 8230. (3) (a) Smothers, W. K.; Wrighton, M. S. J. Am. Chem. Soc. 1983, 105, 1067. (b) Wang, Y. S.; Liu, S. X.; Pinto, M. R.; Dattelbaum, D. M.; Schoonover, J. R.; Schanze, K. S. J. Phys. Chem. A 2001, 105, 11118. (4) Tung, Y. L.; Wu, P. C.; Liu, C. S.; Chi, Y.; Yu, J. K.; Hu, Y. H.; Chou, P. T.; Peng, S. M.; Lee, G. H.; Tao, Y.; Carty, A. J.; Shu, C. F.; Wu, F. I. Organometallics 2004, 23, 3745. (5) Shinozaki, K.; Takahashi, N. Inorg. Chem. 1996, 35, 3917. (6) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H. E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc. 2001, 123, 4304. (7) Adachik, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Appl. Phys. Lett. 2000, 77, 904. (8) (a) Lo, K. K. W.; Lau, J. S. Y.; Fong, V. W. Y. Organometallics 2004, 23, 1098. (b) Li, F.; Zhang, M.; Cheng, G.; Feng, J.; Zhao, Y.; Ma, Y. G.; Liu, S. Y.; Shen, J. C. Appl. Phys. Lett. 2004, 84, 148. (c) Chan, W. K.; Ng, P. K.; Gong, X.; Hou, S. J. Appl. Phys. Lett. 1999, 75, 3920. (d) Gong, X.; Ng, P. K.; Chan, W. K. Adv. Mater. 1998, 10, 1337. (e) Li, F.; Zhang, M.; Feng, J.; Cheng, G.; Wu, Z. J.; Ma, Y. G.; Liu, S. Y.; Sheng, J. C.; Lee, S. T. Appl. Phys. Lett. 2003, 83, 365. (f) Li, B.; Li, M. T.; Hong, Z. R.; Li, W. L.; Yu, T. Z.; Wei, H. Z. Appl. Phys. Lett. 2004, 85, 4786. (g) Walters, K. A.; Kim, Y. J.; Hupp, J. T. Inorg. Chem. 2002, 41, 2909. (h) Dunn, A. R.; Bittner, W. B.; Winkler, J. R.; Getzoff, E. D.; Stuehr, D. J.; Gray, H. B. J. Am. Chem. Soc. 2005, 127, 5169. (9) Machura, B.; Kruszynski, R.; Kusz, J. Polyhedron 2007, 26, 2543. (10) Vogler, A.; Kunkely, H. Angew. Chem., Int. Ed. 1981, 20, 469. (11) Kotch, T. G.; Lees, A. J.; Fuerniss, S. J.; Papathomas, K. I. Chem. Mater. 1992, 4, 675. 3180


The Journal of Physical Chemistry A (12) (a) Machura, B.; Jaworska, M.; Lodowski, P.; Kusz, J.; Kruszynski, R.; Mazurak, Z. Polyhedron 2009, 28, 2571. (b) Machura, B.; Kruszynski, R.; Jaworska, M.; Lodowski, P.; Penczek, R.; Kusz, J. Polyhedron 2008, 27, 1767. (13) Stufkens, D. J.; Vlcek, A., Jr. Coord. Chem. Rev. 1998, 177, 127. (14) Costa, R.; Baone, N.; Gorczycka, C.; Powers, E. F.; Cupelo, W.; Lopez, J.; Herrick, R. S.; Ziegler, C. J. J. Organomet. Chem. 2009, 694, 2163. (15) (a) Linder, K. E.; Wen, M. D.; Nowotnik, D. P.; Malley, M. F.; Gougoutas, J. Z.; Nunn, A. D.; Eckelman, W. C. Bioconjugate Chem. 1991, 2, 160. (b) Linder, K. E.; Wen, M. D.; Nowotnik, D. P.; Ramalingam, K.; Sharkey, R. M.; Yost, F.; Narra, R. K.; Nunn, A. D.; Eckelman, W. C. Bioconjugate Chem. 1991, 2, 407. (16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision B.04; Gaussian, Inc.: Wallingford, CT, 2003. (17) Runge, E.; Gross, E. K. U. Phys. Rev. Lett. 1984, 52, 997. (18) (a) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (b) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1997, 78, 1396. (c) Adamo, C.; Barone, V. J. Chem. Phys. 1999, 110, 6158. (19) (a) Helgaker, T.; Jørgensen, P. J. Chem. Phys. 1991, 95, 2595. (b) Bak, K. L.; Jørgensen, P.; Helgaker, T.; Rund, K.; Jenson, H. J. A. J. Chem. Phys. 1993, 98, 8873. (c) Autschbach, J.; Ziegler, T.; Gisbergen, S. J. A.; Baerends, E. J. J. Chem. Phys. 2002, 116, 6930. (20) (a) Cances, E.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 107, 3032. (b) Cossi, M.; Barone, V.; Mennucci, B.; Tomasi, J. Chem. Phys. Lett. 1998, 286, 253. (c) Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 106, 5151. (21) Shi, L. L.; Liao, Y.; Zhao, L.; Su, Z. M.; Kan, Y. H.; Yang, G. C.; Yang, S. Y. J. Organemet. Chem. 2007, 692, 5368. (22) (a) Stoyanov, S. R.; Villegas, J. M.; Rillema, D. P. Inorg. Chem. 2002, 41, 2941. (b) Di Censo, D.; Fantacci, S.; De Angelis, F.; Klein, C.; Evans, N.; Kalyanasundaram, K.; Bolink, H. J.; Gratzel, M.; Nazeeruddin, M. K. Inorg. Chem. 2008, 47, 980. (c) Kwon, T. H.; Cho, H. S.; Kim, M. K.; Kim, J. W.; Kim, J. J.; Lee, K. H.; Park, S. J.; Shin, I. S.; Kim, H.; Shin, D. M.; Chung, Y. K.; Hong, J. I. Organometallics 2005, 24, 1578. (d) Zhao, Q.; Liu, S. J.; Shi, M.; Wang, C. M.; Yu, M. X.; Li, L.; Li, F. Y.; Yi, T.; Huang, C. H. Inorg. Chem. 2006, 45, 6152. (e) Zheng, K.; Wang, J.; Peng, W.; Liu, X.; Yun, F. J. Phys. Chem. A 2001, 105, 10899. (f) Zheng, K.; Wang, J.; Shen, Y.; Kuang, D.; Yun, F. J. Phys. Chem. A 2001, 105, 7248. (g) Vlcek, A., Jr.; Zalis, S. J. Phys. Chem. A 2005, 109, 2991. (23) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (24) (a) Gabrielsson, A.; Matousek, P.; Towrie, M.; Hartl, F.; Zalis, S.; Vlcek, A. J. Phys. Chem. A 2005, 109, 6147. (b) Dattelbaum, D. M.; Omberg, K. M.; Hay, P. J.; Gebhart, N. L.; Martin, R. L.; Schoonover, J. R.; Meyer, T. J. J. Phys. Chem. A 2004, 108, 3527. (c) Dattelbaum, D. M.; Martin, R. L.; Schoonover, J. R.; Meyer, T. J. J. Phys. Chem. A 2004, 108, 3518. (d) Lundin, N. J.; Walsh, P. J.; Howell, S. L.; McGarvey, J. J.; Blackman, A. G.; Gordon, K. C. Inorg. Chem. 2005, 44, 3551. (25) Busby, M.; Liard, D. J.; Motevalli, M.; Toms, H., Jr.; Vlcek, A. Inorg. Chim. Acta 2004, 357, 167. (26) Ran, X. Q.; Feng, J. K.; Wong, W. Y.; Ren, A. M.; Poon, S. Y.; Sun, C. C. Chem. Phys. 2010, 368, 66.

ARTICLE

(27) (a) Hush, R. A. J. Chem. Phys. 1958, 28, 962. (b) Marcus, R. A. J. Chem. Phys. 1956, 24, 966. (c) Marcus, R. A. Rev. Mod. Phys. 1993, 65, 599. (28) Hutchison, G. R.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 2005, 127, 2339. (29) Li, Y.; Ren, A. M.; Feng, J. K.; Liu, X. J.; Ma, Y. G.; Zhang, M.; Liu, X. D.; Shen, J. C.; Zhang, H. X. J. Phys. Chem. A 2004, 108, 6797. (30) (a) Rodríguez, A. M. B.; Gabrielsson, A.; Motevalli, M.; Matousek, P.; Towrie, M.; Sebera, J.; Zalis, S., Jr.; Vlcek, A. J. Phys. Chem. A 2005, 109, 5016. (b) Li, X.; Liu, X.; Wu, Z.; Zhang, H. J. Phys. Chem. A 2008, 112, 11190. (c) Shi, L.; Hong, B.; Guan, W.; Wu, Z.; Su, Z. J. Phys. Chem. A 2010, 114, 6559. (d) Li, X. N.; Wu, Z. J.; Liu, X. J.; Zhang, H. J. Phys. Chem. A 2010, 114, 9300. (31) Rodríguez, L.; Ferrer, M.; Rossell, O.; Duarte, F. J. S.; Santos, A. G.; Lima, J. C. J. Photochem. Photobiol. A: Chem. 2009, 204, 174. (32) Kamlet, M. J.; Taft, R. W. J. Am. Chem. Soc. 1976, 98, 377.

3181


Ligand Effects on Structures and Spectroscopic Properties of Pyridine

Recommend Documents